39 research outputs found
Monovalent Ion Condensation at the Electrified Liquid/Liquid Interface
X-ray reflectivity studies demonstrate the condensation of a monovalent ion
at the electrified interface between electrolyte solutions of water and
1,2-dichloroethane. Predictions of the ion distributions by standard
Poisson-Boltzmann (Gouy-Chapman) theory are inconsistent with these data at
higher applied interfacial electric potentials. Calculations from a
Poisson-Boltzmann equation that incorporates a non-monotonic ion-specific
potential of mean force are in good agreement with the data.Comment: 4 pages, 4 figure
Vaporization and Layering of Alkanols at the Oil/Water Interface
This study of adsorption of normal alkanols at the oil/water interface with
x-ray reflectivity and tensiometry demonstrates that the liquid to gas
monolayer phase transition at the hexane/water interface is thermodynamically
favorable only for long-chain alkanols. As the alkanol chain length is
decreased, the change in excess interfacial entropy per area decreases to zero.
Systems with small values of excess interfacial entropy form multi-molecular
layers at the interface instead of the monolayer formed by systems with much
larger excess interfacial entropy. Substitution of n-hexane by n-hexadecane
significantly alters the interfacial structure for a given alkanol surfactant,
but this substitution does not change fundamentally the phase transition
behavior of the monolayers. These data show that the critical alkanol carbon
number, at which the change in excess interfacial entropy per area decreases to
zero, is approximately six carbons larger than the number of carbons in the
alkane solvent molecules.Comment: 27 pages, 10 figures, to be published in J. Phys. Cond. Ma
Evolution and Reversible Polarity of Multilayering at the Ionic Liquid/Water Interface
Highly correlated positioning of ions underlies Coulomb interactions between ions and electrified interfaces within dense ionic fluids such as biological cells and ionic liquids. Recent work has shown that highly correlated ionic systems behave differently than dilute electrolyte solutions, and interest is focused upon characterizing the electrical and structural properties of the dense electrical double layers (EDLs) formed at internal interfaces. It has been a challenge for experiments to characterize the progressive development of the EDL on the nanoscale as the interfacial electric potential is varied over a range of positive and negative values. Here we address this challenge by measuring X-ray reflectivity from the interface between an ionic liquid (IL) and a dilute aqueous electrolyte solution over a range of interfacial potentials from −450 to 350 mV. The growth of alternately charged cation-rich and anion-rich layers was observed along with a polarity reversal of the layers as the potential changed sign. These data show that the structural development of an ionic multilayer-like EDL with increasing potential is similar to that suggested by phenomenological theories and MD simulations, although our data also reveal that the excess charge beyond the first ionic layer decays more rapidly than predicted
Molecular ordering and phase behavior of surfactants at water-oil interfaces as probed by X-ray surface scattering
Surfactants have their primary utility, both scientific and industrial, at the liquid-liquid interface. We review recent X-ray surface scattering experiments that probe the molecular ordering and phase behavior of surfactants at the water-oil interface. The presence of the oil modifies the interfacial ordering in a manner that cannot be understood simply from analogies with studies of Langmuir monolayers of surfactants at the water-vapor interface or from the traditional view that the solvent is fully mixed with the interfacial surfactants. These studies explored the role of chain flexibility and head group interactions on the ordering of long-chain alkanols and alkanoic acids. Small changes in the surfactant may produce large changes in the interfacial ordering. The interfacial monolayer can be spatially homogeneous or inhomogeneous. Investigators have observed interfacial phase transitions as a function of temperature between homogenous phases, as well as between homogeneous and inhomogeneous phases. Finally, varying the solvent chain length can alter the fundamental character of the phase transitions and lead to the formation of multilayer interfacial structures